In recent years, magnetic recording apparatuses such as a magnetic disk apparatus, a flexible disk apparatus and a magnetic tape apparatus are widely used with their importance being increasing. Recording density of a magnetic recording medium used in the magnetic recording apparatus is greatly enhanced. Especially, since the development of MR head and PRML technique, the areal recording density is more and more increasing. Recently GMR head and TMR head have been developed, and the rate of increase in the areal recording density is about 100% per year. There is still increasing a demand for further enhancing the recording density, and therefore, a magnetic layer having higher coercive force, and a higher signal-to-noise ratio (SNR) and a high resolution are eagerly desired.
An attempt of increasing the track density together with an increase of a liner recording density to enhance an areal recording density is also being made.
In a recent magnetic recording medium, the track density has reached about 110 kTPI. However, with an increase of the track density, magnetic recording information is liable to inferring with each other between adjacent tracks, and magnetization transition regions in the boundary regions thereof as noise source tend to impair the SNR. These problems result in lowering in bit error rate and impede the enhancement of the recording density.
To enhance the areal recording density, it is required to render small the size of each recording bit and give the maximum saturated magnetization and magnetic film thickness to each recording bit. However, when the bit size is decreased, the minimum magnetization volume per bit becomes small, and the recorded data are tend to disappear due to magnetization reversal caused by heat fluctuation.
Further, in view of the reduction in distance between the adjacent tracks, a high-precision track servo system technology is required for the magnetic recording apparatus, and an operation is adopted wherein recording is carried out widely but the reproduction is carried out narrowly so that the influence of the adjacent tracks is minimized. This operation is advantageous in that the influence of the adjacent tracks can be minimized, but disadvantageous in that the reproduction power is rather low. This also leads to difficulty in enhancement of the SNR to a desired level.
To reduce the heat fluctuation, maintain the desired SNR and obtain the desired reproduction of output, a proposal has been made wherein ridges and grooves are formed on a magnetic recording medium so that each of patterned tracks on the ridges is partitioned by the grooves whereby the track density is enhanced. This type of magnetic recording media is hereinafter referred to as a discrete track media, and the technique for providing this type of magnetic recording media is hereinafter referred to as a discrete track method.
An example of the discrete track medium is a magnetic recording medium disclosed in patent document 1, which is made by providing a non-magnetic substrate having protrusions and depressions formed on the surface thereof, and the magnetic layer corresponding surface configuration is formed on the non-magnetic substrate, to give physically discrete magnetic recording tracks and servo signal patterns.
The magnetic recording medium in JP-A 2004-164692 has a structure such that a ferromagnetic layer is formed via a soft magnetic layer on the non-magnetic substrate having protrusions and depressions formed on the surface thereof, and an overcoat is formed on the ferromagnetic layer. The magnetic recording pattered regions form the protrusions partitioned from the surrounding regions.
In the above-mentioned magnetic recording medium, the occurrence of ferromagnetic domain wall in the soft magnetic layer can be prevented or minimized and therefore the influence due to the heat fluctuation is reduced and the interfere between the adjacent signals is minimized with the result of provision of a magnetic recording medium having a large SNR.
The discrete track method includes two type of methods: a first type is drawn to a method wherein tracks are formed after the formation of a multilayer magnetic recording medium comprising several laminated films; and a second type is drawn to a method wherein patterns having protrusions and depressions are formed on a substrate or formed on a film layer for forming tracks thereon, and then a multilayer magnetic recording medium is made using the patterned substrate or film layer (see, for example, patent document 2 and patent document 3).
The first type discrete track method is often called as magnetic layer-fabricating type method. The fabrication of the surface for the formation of patterned tracks is carried out on the once-formed magnetic recording medium, and therefore, has problems such that the magnetic recording medium is liable to be stained in the fabrication course, and the fabrication process is complicated.
The second type discrete track method is often called as an embossing type method. This method does not have the problem of being stained in the fabrication course, but, has a problem such that the pattern with protrusions and depressions, formed on a substrate, influences the surface configuration of the layers formed thereon, and therefore, it influences the stability in floating state of a floating recording-reproducing head and in height of floatation.
Further, other discrete track methods have been proposed in patent document 4, patent document 5 and patent document 6. In these methods, a previously formed magnetic layer of a magnetic recording medium is, for example, subjected to an implantation of nitrogen ion or oxygen ion or irradiated with laser whereby the magnetic characteristics of regions partitioning magnetic tracks are selectively modified.
However, the above-mentioned discrete tack method utilizing modification of regions partitioning magnetic tracks has a problem such that the magnetic layer tends to be damaged by ion implantation or laser irradiation, and further that, although an ion implantation or laser irradiation with high energy is applied, its energy density over the entire magnetic recording medium is low and the time required for achieving the desired degree of modification is long.    Patent document 1 JP 2004-164692 A1    Patent document 2 JP 2004-178793 A1    Patent document 3 JP 2004-178794 A1    Patent document 4 JP H5-205257 A1    Patent document 5 JP 2006-209952 A1    Patent document 6 JP 2006-309841 A1